1. Field of the Invention
This invention relates to a radiological image pickup apparatus with a radiation detection section of direct conversion type used in a medical field, an industrial field, a nuclear field, etc., and in particular to an art for improving the S/N ratio of the radiological image pickup apparatus.
2. Description of the Related Art
Radiation (for example, X-ray) detectors include those of indirect conversion type wherein radiation (for example, X-rays) is first converted into light and then the provided light is converted into an electric signal by photoelectric conversion and those of direct conversion type wherein incident radiation is converted directly into an electric signal by a radiation-sensitive semiconductor.
The latter radiation detection section of direct conversion type applies a predetermined bias voltage to a voltage application electrode formed on the surface of a radiation-sensitive semiconductor and collects carriers produced with incidence of radiation from a carrier collection electrode formed on the back of the semiconductor and then takes out the carriers as an electric signal, thereby detecting radiation.
Particularly, to use an amorphous semiconductor such as amorphous selenium as a radiation-sensitive semiconductor, the amorphous semiconductor can be easily formed as a thick film of a large area by a method of vacuum evaporation, etc., and thus is fitted for forming a two-dimensional array radiation detection section requiring a large area.
As shown in FIG. 8, a two-dimensional array radiological image pickup apparatus in a related art comprises a radiation detection section having an active matrix substrate 6, a radiation-sensitive semiconductor 7, and a voltage application electrode 8, an LSI chip 9, a signal processing circuit 10 and a flexible wiring film 11. The active matrix substrate 6 is formed with a charge-storage capacitor, a charge read switching element, and a pixel electrode for each unit cell with signal lines and scanning lines disposed like a lattice on the substrate. The radiation-sensitive semiconductor 7 produces charges as radiation is incident and is formed on the active matrix substrate 6. The voltage application electrode 8 is formed on the surface of the semiconductor 7. The LSI chip 9 is formed on the flexible wiring film 11. A two-dimensional radiation detection signal can be obtained by applying a predetermined bias voltage to the voltage application electrode 8 and turning on the switching elements in order for each row by the LSI chip 9 and then reading the charges stored in the charge-storage capacitors for each column through the LSI chip 9 and the signal processing circuit 10.
To use the radiation detection section in FIG. 8, for example, to detect a translucent X-ray image of an X-ray fluoroscopic and radiographic apparatus, a translucent X-ray image can be provided based on the two-dimensional radiation detection signal output from the radiation detection section. To design the two-dimensional array radiation detection section as a compact size, the LSI chip 9 on which charge detection amplifiers and a gate driver are integrated is not separately shielded and is often used in a state in which it is exposed or is only resin-molded. Therefore, the radiation detection section is housed in a conductive cabinet 102 made of metal or a carbon material for use. Thus, it is a common practice to house the radiation detection section in the cabinet for use (JP-A-2002-214352).
However, as shown in FIG. 8, in the radiation detection section of direct conversion type, capacitors are formed between the voltage application electrode 8 and a surface lid section 101 corresponding to a surface lid of the cabinet 102 and when a bias voltage is applied, charges are stored. In this state, if the surface lid section 101 vibrates with vibration of a cooling fan 12, etc., the capacitance of each capacitor fluctuates, so that charge transfer occurs and noise is detected. The surface lid section 101 of the cabinet 102 serves as an incidence window member of radiation and thus needs to be made thin as much as possible and is comparatively easily affected by vibration; it is one of S/N degradation factors.